Applying an inverse symplectic finite Fourier transform (ISFFT) (i.e., an \(N\) -point FFT over the columns and an \(M\) -point IFFT over the rows of \( \mathbf {X}^{\rm {DD}} \) ), we transform the 2-D transmit data block from the delay-Doppler domain to the frequency-time domain [1]}, [2]}, [3]}, [4]}, [5]}
\( \mathbf {X} =  \mathbf {F} _N  \mathbf {X}^{\rm {DD}}  \mathbf {F} _M^H ~, \) 
